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Question

Read the given statements and conclusions carefully. Assuming that the information given in the statements is true, even if it appears to be at variance with commonly known facts, decide which of the given conclusions logically follow(s) from the statements.

Statements:

All polygons are angles.

All angles are diagonals.

All cones are cubes.

All cubes are decagons.

No diagonal is a cube.

Conclusions:

I. Some diagonals are polygons.

II. All diagonals are decagons.

III. No polygon is a cone.

IV. Some cubes are angles.

This question was previously asked in
SSC CGL 2020 Tier-II (English) Previous Year Paper (29-Jan-2022)
The correct answer is

Both conclusions I and III follow

Understanding the Statements and Conclusions Logic

This question requires us to analyze a set of statements and determine which of the given conclusions logically follow from them, assuming the statements are true.

Analyzing the Given Statements

Let's break down the statements provided:

  • Statement 1: All polygons are angles. (Polygon → Angle)
  • Statement 2: All angles are diagonals. (Angle → Diagonal)
  • Statement 3: All cones are cubes. (Cone → Cube)
  • Statement 4: All cubes are decagons. (Cube → Decagon)
  • Statement 5: No diagonal is a cube. (Diagonal ↔ No Cube)

Combining Statements for Logical Deduction

We can combine some statements to find indirect relationships:

  • From Statement 1 and 2: All polygons are angles, and all angles are diagonals. This means All polygons are diagonals. (Polygon → Angle → Diagonal)
  • From Statement 3 and 4: All cones are cubes, and all cubes are decagons. This means All cones are decagons. (Cone → Cube → Decagon)
  • Now consider Statement 5: No diagonal is a cube.
    • Since All polygons are diagonals and No diagonal is a cube, it follows that No polygon is a cube.
    • Since All cones are cubes and No diagonal is a cube, it follows that No cone is a diagonal.
    • Since All cubes are decagons and No diagonal is a cube, it implies the set of diagonals and the set of cubes (and thus decagons related through cubes) are separate.

Evaluating the Conclusions

Let's examine each conclusion based on the deductions from the statements:

  • Conclusion I: Some diagonals are polygons.

    We deduced that All polygons are diagonals. If all of entity A are entity B, then it is logically true that some of entity B are entity A. Therefore, if All polygons are diagonals, then Some diagonals are polygons. This conclusion follows.

  • Conclusion II: All diagonals are decagons.

    We know that No diagonal is a cube and All cubes are decagons. This establishes a separation between diagonals and the set of cubes/decagons. There is no information suggesting that all diagonals must also be decagons. This conclusion does not follow.

  • Conclusion III: No polygon is a cone.

    We established that All polygons are diagonals and No diagonal is a cube. This implies No polygon is a cube. We are also given that All cones are cubes. If no polygon is a cube, and all cones are cubes, then certainly no polygon can be a cone. This conclusion follows.

  • Conclusion IV: Some cubes are angles.

    We know that All angles are diagonals and No diagonal is a cube. This means that the set of angles and the set of cubes are disjoint (they have no elements in common). Therefore, it is impossible for some cubes to be angles. This conclusion does not follow.

Summary of Conclusions

Based on our analysis:

  • Conclusion I: Some diagonals are polygons. (Follows)
  • Conclusion II: All diagonals are decagons. (Does not follow)
  • Conclusion III: No polygon is a cone. (Follows)
  • Conclusion IV: Some cubes are angles. (Does not follow)

Therefore, both conclusions I and III follow from the given statements.

Revision Table: Statements and Inferences

Statements Inference
All Polygons are Angles Polygon → Angle
All Angles are Diagonals Angle → Diagonal
All Polygons are Angles, All Angles are Diagonals ⇒ All Polygons are Diagonals (Polygon → Diagonal)
All Cones are Cubes Cone → Cube
All Cubes are Decagons Cube → Decagon
All Cones are Cubes, All Cubes are Decagons ⇒ All Cones are Decagons (Cone → Decagon)
No Diagonal is a Cube Diagonal ↔ No Cube (Mutually exclusive sets)
All Polygons are Diagonals, No Diagonal is a Cube ⇒ No Polygon is a Cube
All Cones are Cubes, No Diagonal is a Cube ⇒ No Cone is a Diagonal
All Angles are Diagonals, No Diagonal is a Cube ⇒ No Angle is a Cube

Additional Information: Syllogism Rules

This question is based on principles of logical syllogisms, a form of deductive reasoning. Here are some basic rules applied here:

  • All A are B: This means the set of A is a subset of the set of B. From this, it logically follows that Some B are A. It does *not* mean All B are A.
  • No A is B: This means the set of A and the set of B are disjoint (they have no elements in common). From this, it logically follows that No B is A.
  • Transitivity (Chain Rule): If All A are B and All B are C, then it logically follows that All A are C. (A → B → C ⇒ A → C)
  • Negative Chain Rule: If All A are B and No B is C, then it logically follows that No A is C. (A → B, B ↔ No C ⇒ A ↔ No C) Similarly, if No A is B and All B are C, it does not necessarily mean No A is C (A could overlap with C if B is a subset of C). However, if All B are C and No A is B, you cannot say anything definitive about the relationship between A and C without more information. In this problem, we used the rule: All A are B, and No B is C ⇒ No A is C (e.g., Polygons are Diagonals, No Diagonal is Cube ⇒ No Polygon is Cube).

Applying these fundamental rules helps in correctly evaluating whether a conclusion logically follows from the given statements.

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